Process control plants or systems often employ rotary fluid valves, such as ball valves, butterfly valves, eccentric-disk valves, eccentric-plug valves, etc., to control the flow of process fluids. Rotary valves typically include a fluid flow control member disposed in a fluid path and rotatably coupled to a body of the rotary valve via a shaft. Typically, a portion of the shaft extending from the rotary valve is operatively coupled to a shaft of an actuator (e.g., a pneumatic actuator, an electric actuator, a hydraulic actuator, etc.) of the rotary valve.
Certain rotary fluid valves include a visual position indicator to display the angular position of the actuator output shaft or the valve shaft, thereby indicating the position of the fluid flow control member within the valve body. A visual position indicator allows a process engineer or instrument technician to observe valve position and operation without having to connect any additional equipment, such as a laptop or meter, to a valve controller of the valve.
Visual position indicators can be electrical or mechanical. Electrical visual position indicators utilize electronic visual displays, such as LED or LCD displays, to display valve position indicated by an electronic position sensor that is configured to produce an electrical signal in response to changes in the angular position of the valve shaft. Such electronic visual position indicators may be undesirable in certain applications because they require additional power to operate and may be prohibitively expensive. Furthermore, electronic visual position indicators may require operator input or interaction to display position, which may be undesirable for valves installed in areas that are difficult to reach.
Mechanical visual position indicators are directly or indirectly coupled to an actuator output shaft or a valve shaft, and thus do not require additional power to operate. Therefore, mechanical visual position indicators allow process engineers or instrument technicians to observe valve position despite a loss of power. In contrast, electronic visual position indicators merely display the output of the electronic position sensors. Therefore, if the electronic sensors fail, the shaft position is not easily determinable. However, mechanical visual position indicators are directly or indirectly coupled to the actuator output shaft or the valve shaft and operate independently of any electronic position sensors. Therefore, mechanical visual position indicators continue to display shaft position even if the electronic sensors fail. Furthermore, mechanical visual position indicators provide a secondary position indication in addition to the position signal from the electronic position sensor. This secondary position indication may be helpful during setup and calibration phases to validate that the position sensors are operating accurately.
In many applications, it is desirable to minimize power consumption of fluid valve components. Many valves are installed in remote locations, such as oil fields, where external power may not be available. As such, certain valves may operate using energy captured by photovoltaic cells. The cost of photovoltaic cells is often proportional to their energy output. Therefore, fluid valves are designed to minimize power requirements to minimize the size of solar systems needed to power the valves. Thus, mechanical visual position indicators are often desirable because they operate without requiring any additional power.
Many fluid valves are utilized in harsh and/or hazardous environments, and compliance to industry standards related to such environments drives specific design requirements. In certain applications, fluid valves are used in facilities in which the operating environments may include combustible gases, vapors or other compounds. For example, fluid valves are often used in natural gas processing plants or facilities where hydrogen gas is used. The National Electric Code (NEC) in the United States and the Canadian Electric Code (CEC) require that electrical equipment used in hazardous locations carry the appropriate approval from a recognized approval agency. The three main approval agencies in North America are Factory Mutual (FM) and Underwriters Laboratories (UL) in the United States and Canadian Standards Association (CSA) in Canada. Approvals consist of the type of protection and the class, division, groups, and temperature of the operating environment. Certifications commonly used in North America include, for example, explosion-proof, intrinsically safe, non-incendive, and dust ignition-proof.
Electronics within an actuator, for example, may cause a spark that has the potential to ignite a combustible gas or vapor within or surrounding the actuator housing. Explosion-proof valve actuators utilize an enclosure that is capable of withstanding an explosion of a gas or vapor within it and preventing the ignition of a combustible gas or vapor that may surround it. In addition to having sufficient strength to contain an explosion, all passageways connecting the interior of an explosion-proof enclosure to the exterior environment must also provide flame paths to allow the burning gases to escape from the device as they expand during an internal explosion, but only after they have been cooled sufficiently to prevent the ignition of gases or vapors in the external environment. Interfaces defining flame paths must be manufactured to extremely tight tolerances, which may require specialized manufacturing techniques. Furthermore, additional components such as seals may be needed to seal flame paths from the external environment. As such, it is desirable to reduce the total number of interfaces that define flame paths through an enclosure to minimize cost and complexity.
A known visual position indicator comprises a rotary position wheel mounted external to a valve actuator enclosure, opposite a valve shaft. Typically the rotary position wheel is indirectly coupled to the actuator output shaft via additional shafts, gears, cams, and/or other types of coupling members to display the shaft position. The indicator typically includes a polycarbonate shell or cover to protect the rotary position wheel from the external environment. Although this type of position indicator provides a visual position display without requiring electrical power, its location external to the valve actuator enclosure requires an additional opening in the enclosure. For explosion-proof enclosures, this requires machining holes and manufacturing parts to tight tolerances and requires additional seals to seal the interface from the external environment.